Here is a good reference on analytical solutions. You can download it from my link:

https://dl.dropboxusercontent.com/u/109280131/Exact%20solutions%20and%20invariant%20subspaces%20of%20nonlinear%20PDEs%20in%20mechanics%20and%20physics%202007.pdf

You can also try and use Matlab for qualitative analysis. The phase plane analysis can give pretty much idea about the system and stability. So solving them depends on what is the purpose of the equations.

You can use Rk method 

thanks for the reply, but I want to solve it Analytically.

So you can use the perturbaton method 

@ Wakif

Do you have any information about the perturbation method? could you suggest for me some articles or books?

Je vais t'envoyer  quelque liens ichallah

See the work of J.H.He in PRL or my work in Proc.Ind.Nat.Sci.Aca A .

Hope you will get an idea for analytical study.

Prof B.Rath

take a look at those books;

http://bookzz.org/book/441470/0e30ed

http://bookzz.org/book/441474/7d15be

http://bookzz.org/book/463824/f0309b

These are good references. Thanks Abderrahim.

 best regards prof . Yehia A. Khulief 

@ Wakif

Thanks for the referecnes.

@ Maria

I don't think that is possible because every equation has two variables with their time derivative. and for the nonlinear part (i.e. (dx/dt y+x dy/dt)), the separation method failed for this term. 

any other idea?

My suggestion is that see the solution of coupled eqns solved numerically by 

LN  Trefethen using MATLAB. then use data fitting for having an analytical expression.

or see some work on H-H  system in 2D ,preferably in arxiv.

Prof B.Rath

@ Rath

Thank you doctor, the first method sound easy.

for the second method, could you suggest some papers or books?

Read the Eq(35) in arxiv:1112:5294v2

Prof Rath

Dear Friend,

you should apply numerical methods or convert it in system of first order odes and then solve it.

see this article

see also

@Wakif

thanks for the papers, I guess I can use the Homotopy perturbation method for solving my equations.

I'll try it and let you know about it.